Photovoltaic technology offers great potential as an alternative source of electrical energy. Among various types of thin film solar cell technologies, cadmium telluride (CdTe) semiconductor based solar cells have emerged to be one of the leading candidates for large scale, low cost photovoltaic power. The band gap energy of CdTe (1.45-1.5 eV) is well suited for the terrestrial solar spectrum. A few micrometers (μm) of CdTe are sufficient to absorb most of the incident sunlight because of the high optical absorption coefficient thereof. As such, a carrier diffusion length of the order of one micrometer is sufficient to allow all the generated carriers to be collected at the contact electrodes, which significantly relaxes the material quality requirements such as minimum grain size. This is in direct contrast with crystalline silicon solar cells, which require several hundreds of micrometers of silicon owing to the indirect band gap structure thereof. Cadmium telluride, which melts congruently, is the only stable compound between cadmium and tellurium. This advantageous property permits deposition of near stoichiometric CdTe films by various methods including chemical vapor deposition, sputter deposition and close spaced sublimation (CSS). Advances in CdTe technology have led to cell efficiencies of up to 16.5% demonstrated by the researchers from National Renewable Energy Laboratory (NREL).
FIG. 1 is a schematic cross-sectional view of a conventional CdTe thin film photovoltaic device, which includes a glass substrate 21 through which radiant energy or light enters the device; a layer of front contact 23 made of a transparent conductive oxide (TCO) such as fluorine doped tin oxide (SnO2:F) disposed on the substrate 21; an n-type cadmium sulfide (CdS) window layer 25 disposed on the front contact layer 23; a p-type CdTe absorber layer 27 deposited contiguously onto the CdS window layer 25, thereby forming a heterogeneous rectifying junction therebetween; and a layer of back contact 29 made of copper and gold bilayers disposed on the CdTe absorber layer 27.
The photovoltaic device of FIG. 1 has a “superstrate” configuration because the glass substrate 21 is actually on top facing the sun light during operation. The glass substrate 21 is not only used as supporting structure during manufacturing and operation but also as window for transmitting light and as part of the encapsulation. Soda-lime glass, which is limited to a maximum processing temperature of 500° C., is a commonly used substrate material because of its low cost. An alternative substrate material is borosilicate glass which can be processed up to a higher maximum temperature of 600° C. Compared with soda-lime glass, borosilicate glass has a higher transparency, thereby improving current collection, a better matched thermal expansion coefficient to CdTe and fewer impurities which may adversely affect the electrical properties of solar cells.
In fabricating the photovoltaic device illustrated in FIG. 1, the front contact 23 made of a TCO material is first deposited onto the glass substrate 21 by physical vapor deposition (PVD) or chemical vapor deposition (CVD). The TCO of the front contact 23 collects light generated charge carriers while permitting sunlight to pass therethrough. The thickness of the TCO is thus a trade-off between high optical transparency and low sheet resistance. The TCO layer is normally doped to decrease the sheet resistance and is thick enough to form a barrier against diffusion of unwanted species from the glass substrate 21 during operation and high temperature fabrication process. The most commonly used front contact material for CdTe solar cells is fluorine doped tin oxide (SnO2:F) because soda-lime glass pre-coated with SnO2:F is readily available from commercial glass suppliers. The sheet resistance of SnO2:F is sufficiently low enough as not to hinder the solar cell performance. However, SnO2:F has a relatively low optical transmission coefficient of approximately 80%, which limits the short circuit current (Jsc) of the solar cell. Alternative TCO materials such as cadmium stannate (CdSnO4) have also been employed for the front contact 23 to overcome the low optical transmission problem associated with SnO2:F.
An optional secondary TCO layer (not shown) with higher sheet resistance than that of the TCO layer used for the front contact 23 may be inserted between the window layer 25 and the front contact 23. This secondary TCO layer formed on the front contact is known as high-resistance TCO (HRT). Undoped zinc stannate (ZnSnO4), zinc oxide (ZnO) and SnO2 have been employed as the HRT layer which permits thinning of the CdS window layer 25, thereby increasing the quantum efficiency of the solar cell in the blue region. The HRT layer may also improve the solar cell efficiency by forming a barrier against diffusion of unwanted species from the glass substrate 21 and the front contact 23 to the CdS window layer 25.
Following the deposition of the front contact 23 onto the glass substrate 21 of the photovoltaic device illustrated in FIG. 1, a first mechanical or laser scribing step is applied over the entire width of the substrate 21 to pattern the front contact 23 and to isolate individual cells. The cuts have to be as narrow as possible (100 μm range) and are spaced apart by 5-10 mm.
After scribing the front contact layer 23, the n-type CdS window layer 25 having a thickness of 80-300 nm is deposited onto the front contact layer 23 by various methods including sputter deposition, chemical bath deposition and close spaced sublimation (CSS). Since CdS has a band gap energy of approximately 2.4 eV, high energy (blue) photons may be absorbed by the CdS window layer 25 before reaching the CdTe absorber layer 27. Photons absorbed by the CdS window layer 25 do not contribute to the current generation of the device. Consequently, the thickness of the CdS window layer 25 should be as thin as possible to minimize this loss. The thinning of the CdS window layer 25, however, may lead to pin-hole formation, which may cause a shunt between the front contact 23 and the CdTe absorber layer 27. This is further complicated by the fact that some CdS is consumed during the subsequent CdTe deposition and post-deposition annealing described below.
According to the device illustrated in FIG. 1, the p-type CdTe absorber layer 27 having a thickness of 2-8 μm is then deposited onto the n-type CdS window layer 25, thereby forming a heterogeneous p-n junction. There has been a trend to minimize the thickness of the CdTe absorber layer 27 in order to conserve the rare element tellurium (Te), because it is thought that there may not be adequate Te supply to fabricate enough CdTe photovoltaic devices to meet global demand. Cadmium telluride has a direct band gap energy of 1.45-1.5 eV, which is well suited for the terrestrial solar spectrum for a single junction solar cell. Moreover, the direct band gap structure gives CdTe a high optical absorption coefficient of 5×104 cm−1 in visible light spectrum. One micrometer of CdTe can absorb 92% of incident sunlight.
While there are a multitude of methods for depositing the CdTe absorber layer 27, close spaced sublimation (CSS) and variants thereof are the preferred methods for depositing CdTe in manufacturing of CdTe photovoltaic device. In the CSS method, a substantially flat substrate is positioned in parallel with a CdTe source plate along a plane spaced apart therefrom in a rough vacuum environment. The substrate and the source plate are heated to about 500° C. with the substrate temperature being held at a slightly lower temperature, which drives the condensation of CdTe vapor on the substrate surface and thus CdTe film formation on the same.
After the deposition of the CdTe absorber layer 27, a layer of cadmium chloride (CdCl2) is deposited onto the surface thereof, which is then followed by a 20 to 30 minute annealing at about 450° C. in air. The CdCl2 may be deposited using a vacuum deposition technique or a colloidal solution containing a mixture of CdCl2 particles and methanol. The CdCl2 treatment can also be applied during the deposition of the CdTe absorber layer 27. The need for the CdCl2 treatment arises from the difficulties in doping CdTe with high concentration of foreign dopants because of the ability of native defects in CdTe to form complexes, which can act as doping centers themselves and potentially compensate for the impurity doping. While the precise mechanisms of CdCl2 treatment are still not fully clear, it is generally believed that a chlorine (Cl) atom from CdCl2 substituting a Te atom acts as a shallow donor, thereby increasing p-type doping of CdTe.
Following the CdCl2 treatment, a second mechanical or laser scribing step is applied over the entire width of the substrate 21 to pattern the interconnect between adjacent cells of the photovoltaic device illustrated in FIG. 1. The cuts stop on the front contact 23 and are spaced apart by 5-10 mm depending on the design.
After the second scribing step to pattern the interconnect between adjacent cells, a layer of back contact 29 is deposited onto the CdTe absorber layer 27. In order to establish a low resistive ohmic contact with the p-type CdTe semiconductor, the work function of the metallic contact material needs to exceed that of the semiconductor. However, the p-type CdTe has an extremely high work function of 5.8 eV owing to high electron affinity thereof, which essentially prevents the formation of a low resistive CdTe/metal contact for all known metals. If the work function of the contact does not exceed that of the p-type CdTe, a Schottky barrier would form at the interface which would hinder the carrier movement across the same, thereby decreasing the device efficiency.
There are several schemes developed to minimize the contact barrier at the interface between the back contact 29 and the CdTe absorber layer 27. The implementation of these schemes is the primary reason for fabrication CdTe photovoltaic device in the “superstrate” configuration. One approach to overcome the above mentioned problem is to minimize the width of the barrier by increasing the dopant concentration of CdTe at the contact/CdTe interface, thereby permitting carriers to tunnel therethrough. The dopant concentration may be increased by selectively etching Cd to form a Te-rich surface. Another approach is to form a thin, intermediate layer such as copper telluride (CuxTe) and copper doped zinc telluride (ZnTe:Cu) in between the contact and CdTe. As such, the deposition of the back contact 29 is often implemented as a bilayer system: a thin layer of copper is first deposited on the CdTe surface to form copper telluride and then followed by the deposition of a thicker conductive material, such as gold, titanium and graphite-silver paste.
Following the deposition of the back contact 29, a third mechanical or laser scribing step is applied over the entire width of the substrate 21 to pattern the back contact 29, thereby completing the photovoltaic device illustrated in FIG. 1.
Much of the development effort in CdTe solar cell heretofore has been focused on the CdCl2 treatment and the back contact schemes described above. While the band gap energy of CdTe is considered to be nearly ideal for a solar cell with a single absorber layer, CdTe does not absorb photons with energies less than its band gap energy of 1.45 eV. Unlike crystalline silicon and copper indium gallium selenide (CIGS) photovoltaic devices which have lower band gap energies of about 1.1 eV to 1.2 eV and higher conversion efficiencies, CdTe photovoltaic device cannot effectively convert the red and near infrared part of the sunlight into electrical energy.